The present invention relates to a shadow mask type color cathode ray tube.
Generally, a color cathode ray tube comprises an envelope including a substantially rectangular panel in which a skirt portion is provided on a periphery of an effective portion thereof, and a funnel connected to the skirt portion. On the inner surface of the effective portion of the panel is formed a phosphor screen comprising three-color phosphor layers. A shadow mask is disposed inside of the phosphor screen so as to oppose it. An electron gun for emitting three electron beams is disposed in a neck of the funnel. Then, the three electron beams emitted from the electron gun are deflected by a magnetic field generated from a deflection apparatus provided outside of the funnel, and vertically and horizontally scan the phosphor screen through the shadow mask, thereby displaying color image on the phosphor screen.
The shadow mask is used for selecting three electron beams and comprises a substantially rectangular mask body in which a number of electron beam passage apertures are formed on a region opposing the phosphor screen and a substantially rectangular mask frame having side walls fixed to the periphery of the mask body. As a shadow mask supporting method, according to some type thereof, substantially wedge shaped elastic supports are provided at corners of the mask frame and these elastic supports are engaged with stud pins provided at the skirt portion of the panel, so that the shadow mask is detachably supported thereby.
Generally, in order to display color picture images having no deviation in color on the phosphor screen of the color cathode ray tube, three electron beams which pass the electron beam passage apertures in the mask body must be landed properly on the corresponding three color phosphor layers. For this purpose, it is necessary to keep the shadow mask at a proper position relative to the panel.
To meet recent trend of multi-media, there is provided a color cathode ray tubes in which the size of three color phosphor layers and their arrangement pitch are reduced to increase its resolution. In such a color cathode ray tube, allowance to deviation of beam landing relative to the three color phosphor layers has dropped so that more accurate beam landing than ordinary color cathode ray tubes is demanded.
However, in conventional color cathode ray tubes, the shadow mask is so constructed that almost entire periphery of the mask body is in contact with the side walls of the mask frame. Thus, there sometimes occurs a case in which the shadow mask may be deviated from its proper position because a undesired force is applied to the mask body due to disparity in quality of the mask frame resulting from production. Particularly in recent color cathode ray tubes having aspect ratio of 16:9, the side wall on the long side of the mask frame is much longer than the side wall on the short side as compared to ordinary color cathode ray tubes. Thus, the mask body is likely to be deformed because of insufficient strength and a disparity in distance between the opposing side walls on the long side (distance in the direction of the short axis).
In manufacturing process for the color cathode ray tubes, fitting and removal of the shadow mask are carried out repeatedly. Due to a force applied to the shadow mask during this fitting and removal operation, a distortion may occur in the mask frame so that the mask body may be deformed. If such a phenomenon occurs, a deviation in beam landing to the three color phosphor layers is produced.
On the other hand, in color cathode ray tubes for the multi-media, which have a small allowance in beam landing to the three color phosphor layers, it is necessary to reduce a load on the elastic supports by reducing the weight of the mask frame occupying most part of the weight of the shadow mask so as to relax the deviation of the shadow mask due to external shock. However, if the weight of the mask frame is reduced, it is likely that the mask frame is deformed due to an external shock or a force applied when the shadow mask is fitted or removed so that the shadow mask is deviated relative to the panel.
Further, although such a procedure has been taken as to reduce the thickness of the mask body thereby equalizing the size and shape of the electron beam passage apertures which are a prominent reason for generation of unevenness of the phosphor screen, reducing the thickness of the mask body likely induces a reduction of the strength thereby producing a deformation of the mask body itself.
There have bee provided a color cathode ray tube employing a low thermal expansion type shadow mask which is made of amber material having a high effect of suppressing a transition of electron beams due to so-called doming that the shadow mask is expanded in a direction of the phosphor screen due to thermal expansion of the shadow mask. As a measure for preventing a deformation of such a low thermal expansion type shadow mask, Jpn. Pat. Appln. KOKAI No. 5-121009 has disclosed a shadow mask in which a plurality of protruding portions which protrude outwardly are provided on side walls of the mask frame. However, even in the case in which such protruding portions are provided, since the mask body and the mask frame are in contact with each other through planes, a deformation of the mask body is generated due to disparity in production of the mask frame or deformation of the mask frame in a manufacturing process of color cathode ray tubes.
As described above, design of the mask frame is important in order to display picture images having no deviation in color on the phosphor screen. However, conventional shadow masks have such a problem that the mask body or mask frame is deformed and it is difficult to display high quality images.